1. Field of the Invention
The present invention relates to a communications method and communications system, and particularly to demand-assigned satellite communications systems which provide voice, facsimile, data and other digital transmission services.
2. Description of the Related Art
FIG. 1 is an example of a network configuration showing a communications system according to a conventional system. In FIG. 1, reference numeral 20 is a communications satellite. The satellite generally comprises a receiver receiving transmitted signals from earth stations, a frequency converter for converting frequencies, and a transmitter for transmitting to earth stations. The earth stations comprise a network control station 21, an earth station 22, and mobile stations 23.
FIG. 2 is a block diagram showing the structure of the network control station 21. In FIG. 2, an antenna 31 is controlled to the direction of the satellite 20 by a control servo loop which comprises a tracking signal wave dividing unit 35, a tracking receiver 34, an angle detector 32 and an antenna driving unit 33.
In FIG. 2, the control signal received from the antenna 31 is sent to a transmission/receive duplexer 36 and obtained as a control receive signal. The control receive signal is sent to a low noise amplifier 37. The resultant signal is sent to a frequency converter 38. The frequency converter 38 converts the frequency of the amplified signal into an intermediate frequency. The resultant signal is sent to a demodulator 39. The demodulator 39 demodulates a TDMA (Time Division Multiple Access) control signal. An output signal of the demodulator 39 is sent to a controlling circuit 40. The controlling circuit 40 searches for a proper channel and converts data thereof into a control signal, for example, a call request or a call response. The control signal is sent to a modulator 41. The modulator 41 modulates the output signal, for example, an assigned channel or the like, of the controlling circuit 40. The resultant signal is sent to a frequency converter 42. The frequency converter 42 converts the output signal of the modulator 41 into a control radio frequency signal. The resultant signal is sent to a high power amplifier 43. The high power amplifier 43 amplifies the power of the radio frequency signal. The resultant signal is sent to the antenna 31 through the transmission/receive duplexer 36 and the tracking signal dividing unit 35. The resultant signal is sent from the antenna 31 to the communication satellite 20.
In the above conventional construction, further, the demand assigned communication system presents the following situation.
A demand assignment FDMA (Frequency Division Multiple Access) satellite communications system comprises a network control station that manages and controls a communications system, earth stations connected to a public switched telephone network (PSTN) and/or private networks and mobile stations/mobile terminals from which users access the satellite communication network.
The network control station monitors the entire network. In addition, the network controlling station successively sends channel control information that has been time-division multiplexed to a satellite using a forward link (also referred to as a forward channel or an outbound channel). The time-division multiplex (TDM) information includes information of network status, network control signals and call announcements. An earth station and/or a mobile station/terminal receives the channel control information that has been sent as the TDM information from the network control station. Corresponding to the channel control information, the earth station and/or the mobile station/terminal performs a call request and/or a call response using a return link (also referred to as a return channel or an inbound channel). A call request and/or a call response is sent in predetermined time slots from the earth station and/or the mobile station/terminal based on time division multiple access (TDMA) method corresponding to the TDM network control information.
When the network control station receives a call request or a call response from an earth station or a mobile station/terminal, the network control station informs the earth station and the mobile station/terminal of an available communication channel through the signalling channel. When the earth station and the mobile station/terminal has received the assigned communication channel information, it sets the assigned channel and switches from the signalling channel to the communication channel. With a predetermined process sequence, the mobile station/terminal is connected to the earth station by PSTN or a private service network through the satellite.
In a mobile satellite communications system that communicates with a mobile station installed in a car, a truck, or the like, due to receive signal fading by buildings and other obstacles on a transmission pass (namely, the shadowing between the mobile station and the satellite), a call request and/or a call response may not be sent from the mobile station to the earth station. To solve such a problem, several methods have been proposed. As a first method (referred to as A method), the call request or the call response is repeatedly sent for a predetermined number of times assigned as a parameter of the system. As a second method (referred to as B method), after a mobile station has sent a call request or a call response to the network control station, if relevant control information is not received in a predetermined time period, the call request or the call response is resent.
In the A method, the number of times that the call request or the call response is sent increases. Further, the probability of a transmission burst being blocked by shadowing decreases, but if plural transmission bursts are continuously sent, the probability of receiving the call request or the call response increases. Thus, the possibility of the call being successfully connected increases.
In the B method, after a mobile station has sent a call request or a call response to the network control station, if the network control station does not detect the call request or the call response and an earth station does not recognize it, the mobile station repeatedly sends the call request or the call response at a predetermined interval. Thus, the probability of the network control station detecting the call request or the call response and the earth station receiving it increases.
However, in the A method, although the success rate of the call connection increases, since the same information is repeatedly sent, the transmission efficiency deteriorates. In addition, the power consumption of the mobile station increases. Moreover, since the access amount of the mobile station increases, the probability that call requests and call responses of various mobile stations conflict increases. Thus, in the A method, the call connections are not effectively performed.
On the other hand, in the B method, after a mobile station has sent a call request or a call response to a satellite, if the network control station does not recognize the call request or the call response, the mobile station repeatedly sends the call request or the call response at a predetermined interval. Thus, in the B method, since the mobile station has to wait, it takes a long time to perform a network connection.